#i feel like the difference here is#do you believe that it is statistically likely that life exists somewhere else in the universe?#vs#do you think pod people are secretly controlling our government#and dive bombing farmers with their spaceships#which#i mean#i feel like there is a line the size of the grand canyon between the two

The thing I find really interesting about this is how many people – including highly numerate scientists – agree with the “extraterrestrial life is statistically likely” idea. Because I don’t agree with it.

I appreciate the enormity of space, and the magnificent huge-os-i-ty of the number of potentially habitable planets. But I happen to believe, in all seriousness, that “the notion that in this infinitely-huge universe our planet is the only one with life on it” is not fucking absurd. From my sense of the available data, it’s not possible to assert any particular likelihood for or against that idea.

Because we just don’t know. We don’t have a meaningful sample that would allow us to put a value on that number.

To argue that we can put a value on that number, by arguing that the probability is greater than .5 by saying “it is statistically likely that life exists somewhere else in the universe”, is to focus only on the convenient part of the equation, while ignoring the inconvenient part. But math doesn’t work that way.

Here’s the inconvenient part: we don’t know how likely or unlikely it is that given a planet with conditions suitable for life, life will actually emerge there. We have direct evidence that this has happened once (and only once) here on earth. We have no evidence (so far) that it has ever happened any other time, on earth or anywhere else. It could be a commonplace occurrence. It could be a rare occurrence. It could be so extraordinarily rare that it only has ever happened once in the entire history of the universe.

Now, I’ll grant you that that seems, on the face of it, like an absurd thing to say. But that sense of absurdity is not rooted in mathematics. It’s rooted in our everyday experience here on earth, in which life appears to be commonplace, and things that we see happening on occasion, even extraordinarily unlikely things like winning the lottery, are nevertheless likely enough that we do see them happening once in a while, given enough throws of the dice. And so, since the universe is so unimaginably huge, we have an intuitive sense that surely the odds of rolling “life” with that many rolls of the dice must be greater than 50%. Because again, look around us: we rolled it here on earth. Tada! says our intuitive sense.

But in this case our intuitive sense is being an ass.

The universe is under no obligation to have any particular value for the likelihood of life’s emergence. Well, okay, that’s not quite true: It’s obligated to have a number greater than zero, because we’re here to ask the question. But that’s the universe’s only obligation in that regard. Life’s emergence given suitable conditions could be likely. It could be unlikely. It could be unimaginably unlikely, even so unlikely as to make it so our own emergence was the only time it ever happened despite the unimaginably large number of earth-like worlds in the cosmos.

We could totally be living in that universe, the one in which life emerged just once. Just as we could totally be living in a universe in which life emerged more times than that. As long as our sample of life having emerged consists only of the single case required by us being here to ask the question, there is no mathematical reason to argue for one scenario over the other.

I’m not saying NASA should stop making the search for extraterrestrial life a key part of their fundraising appeal. I think it’s a super interesting question, one we can only answer by going out and looking. I just don’t agree that we actually know, at all, how likely it is that the search is ever going to pay off.

As long as we’ve been looking at the heavens, we’ve operated with this assumption that extraterrestrial life is out there. As technology has extended our senses we’ve had to move that imagined life farther away, but we keep imagining it. But that’s because we want it to be true, not because we’ve run the numbers and determined that it is likely that it is.

We (including scientifically minded folks) used to believe we would find life on the Moon. These days we get excited about the possibility of finding it in places like Mars, Titan, or Europa. If the solar system eventually turns up empty we’ll focus our hopes on habitable-zone extrasolar planets. But we’ll continue to imagine it being out there, even as we continue to accumulate negative evidence. It just makes sense to us. It’s what we’re familiar with.

I’m fine if people want to believe there is life beyond earth. I just don’t think they’re using math to decide that, or that they can back up the assertion that it is likely.

Nearly Every Star Hosts at Least One Planet

“The vast majority of stars in our Milky Way galaxy host planets, many of which may be capable of supporting life as we know it, a new study suggests.

Astronomers have detected eight new exoplanet candidates circling nearby red dwarf stars, which make up at least 75 percent of the galaxy’s 100 billion or so stars. Three of these worlds are just slightly bigger than Earth and orbit in the “habitable zone,” the range of distances from a parent star where liquid water could exist on a planet’s surface.

The new finds imply that virtually all red dwarfs throughout the Milky Way have planets, and at least 25 percent of these stars in the sun’s own neighborhood host habitable-zone “super-Earths,” researchers said.”

There’s got to be life out there, if not elsewhere in our own solar system.

Unpopular opinion after the cut.

First: bobak is (rightly) a personal hero to me and geeky people like me everywhere. That’s not the unpopular opinion. That’s fact.

The unpopular opinion is that I think he’s wrong to draw the conclusion he does. That is, he’s wrong to conclude that the large number of habitable-zone planets we know to exist in the universe means “there’s got to be life out there.” It does not actually mean that.

We want it to mean that. People have always wanted to believe there’s some sort of purposeful agency beyond Earth. They used to populate the beyond with gods. Then science came along, and they began populating it with extraterrestrial life. But it’s still just drawing outré sea monsters in the white areas of the map beyond where we’ve actually been.

The problem with the position that Bobak (whom I don’t mean to pick on; most people who’ve expressed an opinion on the question seem to fall into the same fallacy) is espousing is this: We don’t know the Drake equation’s term for F-sub-l, the probability that, given a planet in the “habitable zone” where liquid water can occur, life will actually emerge there. Not just “we don’t know F-sub-l exactly.” We don’t know it at all.

Granted: the number of planets where life broadly similar to life on Earth might emerge is very, very large. The problem is that the probability of life emerging in such a place might be very, very small — even small enough as to make it so the one case of life emerging on Earth is the only case of life ever having emerged in all the universe, or at least in all the universe near enough to be reasonably investigated by us or our our descendants, which for practical purposes is the same thing.

People try to argue that it’s reasonable to assume that the value of F-sub-l is not so small as to make life’s emergence very unlikely. Just look, they say: Life emerged here on Earth. And it apparently emerged relatively soon after the planet’s crust solidified.

That’s true. But it’s also true that all the available evidence indicates that it emerged only once. That is, that abiogenesis only happened one time on Earth, or that if it happened more than once, that the record of all the other times besides the one that eventually led to us left no traces we’ve been able to discover.

The problem with trying to make an inductive case from that single act of abiogenesis is that, per the anthropic principle, the fact of life having emerged that one time doesn’t speak at all to the question of how likely that emergence was. Life may have been likely to emerge. It may have been extraordinarily unlikely to emerge. In either case, it needs to have happened that one time in order for us to even be here considering the question.

You can think about it like this: Say you come upon a multi-sided die, like those 20-sided dice that D&D players use, except this die has a very large number of sides: hundreds of sides, millions of sides, billions of sides; you don’t know. All you know is that when you happened upon it, the die was resting on the ground with the side marked with the numeral 1 facing up.

The question you need to answer now is, if you roll the die again a given number of times, how likely is it that you will roll another 1? Is it likely to the point of near-certainty? Unlikely to the point of near-impossibility? Or somewhere in between?

In this thought experiment, rolling a 1 means abiogenesis happens. What Bobak is basically arguing is that if he knows the number of rolls he gets to make is very large (that is, if the number of life-suitable planets is very large), then it is a near-certainty that he will be able to roll another 1.

But that’s not rational. Because for any number of die rolls, even a very large number of them, I can pick a value for the number of sides on the die that is large enough that even with that many rolls, the chances of rolling another 1 are so small as to be a near-impossibility.

It doesn’t matter that we came upon the die with a 1 already showing. That’s not an independent data point. It’s a necessary precondition.

I absolutely think we should keep looking for life beyond Earth. I just don’t think we should kid ourselves about what we do and don’t know about the likelihood we’ll find it.